Protein quality control operates in cells to limit levels misfolded proteins. It is critical for normal cell health and underlies many diseases. A key branch of quality control (QC) involves ubiquitin-mediated proteolysis of misfolded proteins. We have recently discovered that the highly conserved E3 ubiquitin ligase Ubr1 has a previously unknown role in cytoplasmic quality control, by which aberrant proteins are degraded by Ubr1- mediated ubiquitination. Ubr1 is well-known for its role in the N-end rule, but the newly discovered QC functions of Ubr1 operate independently of these previously described actions. In cytoplasmic quality control, Ubr1 uses a chaperone-dependent mechanism to specifically ubiquitinate a wide variety of misfolded proteins. Furthermore, the Ubr1 QC pathway is physiologically relevant, since it is needed to survive proteotoxic cellular stress. In the proposed studies, we will completely characterize the action and role of Ubr1 in protein quality control, using genetic, molecular biological, and biochemical approaches. The studies span a range of questions, including the variety and characteristics of misfolded protein substrates, the features of Ubr1 required for its quality control function, the role of chaperones on the pathway, and the physiological functions Ubr1-mediated quality control the living cell. Our work on Ubr1-mediated quality control is medically relevant due to the importance of quality control in the etiology of some our society's pressing maladies, and because UBR ligases are highly conserved in all eukaryotes. Loss of UBR ligase functions in mammals causes many detrimental phenotypes, including the mutations of human UBR1 responsible for the severe Johanson-Blizzard syndrome. We believe that the quality control actions of UBR ubiquitin ligases will underlie many aspects of their function in all organisms. Our studies will define those functions and eventually the ways to harness or quell them for basic and biomedical benefit.